Botany · Ch 10 — Reproduction in Organisms
The Zygote
The Zygote
The Zygote
The formation of a diploid zygote by the fusion of gametes is a universal feature of sexual reproduction, found in every sexually reproducing organism. Where the zygote actually forms depends on where fertilisation itself took place: in organisms with external fertilisation the zygote forms in the surrounding medium (typically water), while in organisms with internal fertilisation it forms inside the body of the organism.
What Happens to the Zygote Next
How the zygote develops further depends on the type of life cycle the organism follows and the conditions it experiences. In many fungi and algae, the zygote secretes a thick, resistant wall that protects it against desiccation and physical damage, and it then passes through a resting period before it eventually germinates. In organisms with a haplontic life cycle, the zygote does not simply grow directly into a new individual — instead it divides by meiosis to produce haploid spores, and these spores are what grow into new haploid individuals. Organisms with diplontic or haplo-diplontic life cycles handle the zygote differently again, developing it more directly (or through an intervening multicellular haploid stage) into the mature body form.